The invention relates to the breeding and selection of potatoes.
Apart from being an important staple food, potato is classically the raw material for industrial production of starch from potato tubers. Furthermore, these days, the industrial harvest of potato protein from potato tubers, earlier seen as a quantité négligeable attracts more attention considering the increased value given to vegetable protein sources, for example for animal food, if only as a side product accompanying starch production. Where, chemically, potato starch in potato tubers essentially consists of two components: amylopectin and amylose in a proportion of approximately 80% to 20%, potato proteins in tubers essentially consist of protease inhibitors that help protect the tuber against disease such as parasite infestations or fungal or bacterial rot and storage proteins such as patatin in a proportion of approximately 60% to 40%.
For various reasons, starch producers prefer potatoes with different ratios of amylopectin and amylose. An earlier induced gene mutation in potatoes that affects the synthesis of the enzyme granule bound starch synthase (GBSS), and the subsequent molecular cloning of this gene (Hovenkanp-Hermelink et al., 1987, Theor. Appl. Genet. 75:217-221; Visser et al., 1989, Plant Science 64:185-192) has opened possibilities for altering the starch composition of potatoes—either through established breeding methods or through modern techniques of genetic manipulation.
The GBSS mutation in potato is similar to the so-called waxy (wx) mutation in maize and prevents the production of amylose, when expression or specific function of the GBSS protein is absent. Therefore, this mutation has been designated as amylose-free (amf) mutant of potato. Herein, the amf-gene mutation stands for a modification of the GBSS-gene that leads to a complete functional loss of GBSS-activity, notwithstanding that GBSS-like gene products, without the specific activity, may still be expressed from the gene's transcripts in question, whereby the Amf-gene stands for a gene from which gene products with GBSS-activity can still be obtained. The amf-gene character is determined by a monogenic mendelian recessive gene, the phenotype of which can be detected in various plant parts such as columella cells of root tips, tubers, plastids in the stomatal guard cells and in microspores (Jacobsen et al., 1989, Euphytica 44:43-48). When these parts are stained with a potassium iodine solution (Lugol), starch is stained red in mutants and dark blue in the wild type.
Unlike many other phenotypic genetic markers, the mutated or functionally deleted GBBS- or amf-gene offers certain special advantages for genetic analysis as well as for breeding. For example, the progeny can be classified at a very early seedling stage as well as in adult plants, through pollen staining, homo- and heterozygotes can be unambiguously classified: the dosages 2-4 of the mutant allele in a tetraploid can be easily detected through the ratios 5:1, 1:1 and 0:1 in stained pollen samples; different types of 2n-gametes in diploid clones can be detected and their influence on the phenotype and genotype of tetraploid from 4x*2x crosses can be predicted.
Prospects of using the material in conventional as well as in analytic breeding of potato have since the development of the amf-gene potato mutant of Hovenkamp-Hermelink been opened. A disadvantage for breeding is the recessive nature of amf, which complicates the combination of this character with other agronomic traits at the tetraploid level.
Therefore, the analytic breeding method advocated by Chase (1963, J. Genet. Cytol. 5:359-364), which involves breeding of potato at the diploid level and returning to the tetraploid condition through the use of 2n-gametes, could be of considerable value for breeding amf-varieties. The aim of such investigations are at least two fold: a. to combine amfamf and Amfamf genotypes with that of 2n-gamete formation, and b. to create fertile, nulliplex clones as basic material for breeding amylose-free potatoes. On the other hand, development of suitable diploid material that produces high frequencies of 2n-pollen and 2-eggs would also open the way for unilateral and bilateral sexual polyploidization (Mendiburu and Peloquin, 1976, Theor. Appl. Genet. 48:137-143). Such diploid breeding material may be homozygous (amfamf) or heterozygous (Amfamf), because in both cases selection can be carried out based on pollen phenotype.